Vehicle propulsion system with multi-channel dc bus and method of manufacturing same

ABSTRACT

An apparatus includes a multi-channel DC bus assembly comprising a first channel and a second channel, a first electromechanical device coupled to a positive DC link of the first channel, and a second electromechanical device coupled to a positive DC link of the second channel. A first DC-to-AC voltage inverter is coupled to the positive DC link of the first channel and a second DC-to-AC voltage inverter is coupled to the positive DC link of the second channel. The apparatus further includes a bi-directional voltage modification assembly coupled to the positive DC link of the second channel and a first energy storage system electrically coupled to the first electromechanical device.

BACKGROUND OF THE INVENTION

Embodiments of the invention relate generally to electric drive systemsincluding hybrid and electric vehicles and, more particularly, totransferring energy between one or more energy storage devices andmultiple electromechanical devices of the vehicle using a multi-channelDC bus.

Purely electric vehicles use stored electrical energy to power anelectric motor, which propels the vehicle and may also operate auxiliarydrives. Purely electric vehicles may use one or more sources of storedelectrical energy. For example, a first source of stored electricalenergy may be used to provide longer-lasting energy while a secondsource of stored electrical energy may be used to provide higher-powerenergy for, for example, acceleration.

Hybrid electric vehicles may combine an internal combustion engine andan electric motor powered by an energy storage device, such as atraction battery, to propel the vehicle. Such a combination may increaseoverall fuel efficiency by enabling the combustion engine and theelectric motor to each operate in respective ranges of increasedefficiency. Electric motors, for example, may be efficient ataccelerating from a standing start, while combustion engines may beefficient during sustained periods of constant engine operation, such asin highway driving. Having an electric motor to boost initialacceleration allows combustion engines in hybrid vehicles to be smallerand more fuel efficient.

While propulsion system configurations for purely electric vehicles andhybrid electric vehicles have been developed to include multiple sourcesof electrical energy to increase energy or power density and multiplepower sources to achieve desired propulsive output, incorporating theseenergy storage and power sources into a propulsion system increases theoverall size, weight, and cost of the system. Further, the limitationsimposed by configuring a propulsion system to operate with multiplepower sources in combination with one or more energy storage sourcesreduces the operating efficiency and fuel economy of the individualcomponents of the propulsion system in addition to reducing the overallsystem efficiency.

Therefore, it would be desirable to provide an electric and/or hybridelectric propulsion system that incorporates multiple electromechanicaldevices and one or more energy storage systems in a manner that improvesoverall system efficiency and permits the individual components of thepropulsion system to be operated independently to improve the individualoperating efficiencies thereof.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, an apparatus includes amulti-channel DC bus assembly comprising a first channel and a secondchannel, a first electromechanical device coupled to a positive DC linkof the first channel, and a second electromechanical device coupled to apositive DC link of the second channel. A first DC-to-AC voltageinverter is coupled to the positive DC link of the first channel and asecond DC-to-AC voltage inverter is coupled to the positive DC link ofthe second channel. The apparatus further includes a bi-directionalvoltage modification assembly coupled to the positive DC link of thesecond channel and a first energy storage system electrically coupled tothe first electromechanical device.

In accordance with another aspect of the invention, a method offabricating a propulsion system includes coupling a first DC-to-ACvoltage inverter to a first voltage bus, coupling a firstelectromechanical device to the first DC-to-AC voltage inverter, andcoupling a second DC-to-AC voltage inverter to a second voltage bus. Themethod also includes coupling a second electromechanical device to thesecond DC-to-AC voltage inverter, coupling a bi-directional DC-DCvoltage converter to the second voltage bus, coupling a first energystorage system to the bi-directional DC-DC voltage converter, andprogramming a controller to control switching of the bi-directionalDC-DC voltage converter to boost a voltage of the first energy storagesystem to a boosted voltage different from a voltage of the firstvoltage bus.

In accordance with yet another aspect of the invention, a vehiclepropulsion system includes a DC bus assembly having a first DC bus and asecond DC bus. The vehicle propulsion system also includes a firstbi-directional DC-to-DC voltage converter coupled to the first DC bus, ahigh specific-power energy storage device coupled to a low voltage sideof the first bi-directional DC-to-DC voltage converter, a firstelectromechanical device coupled to the first DC bus through a firstDC-to-AC voltage converter, and a second electromechanical devicecoupled to the second DC bus through a second DC-to-AC voltageconverter. A controller is programmed to control the firstbi-directional DC-to-DC voltage converter to boost a voltage of thefirst electromechanical device to a boosted voltage and supply theboosted voltage to the first DC bus, the boosted voltage different thana voltage of the second DC bus.

In accordance with yet another aspect of the invention, a vehiclepropulsion system includes a first electromechanical device coupled to apositive DC link of a first DC bus, an auxiliary load coupled to anoutput of the first electromechanical device and a first DC-to-ACvoltage inverter coupled to the positive DC link of the first DC bus. Asecond electromechanical device is coupled to a positive DC link of asecond DC bus and a transmission is coupled to an output of the secondelectromechanical device. The vehicle propulsion system also includes asecond DC-to-AC voltage inverter coupled to the positive DC link of thesecond DC bus, an energy storage system electrically coupled to thepositive DC link of the second DC bus, and a bi-directional voltagemodification assembly coupled to the positive DC link of the second DCbus and configured to boost a voltage of the first energy storage systemto a voltage different from a voltage of the first DC bus.

Various other features and advantages will be made apparent from thefollowing detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate embodiments presently contemplated for carryingout the invention.

In the drawings:

FIG. 1 is a schematic diagram of a propulsion system according to anembodiment of the invention.

FIG. 2 is a schematic diagram of a propulsion system that includes anauxiliary load according to an embodiment of the invention.

FIG. 3 is a schematic diagram of a propulsion system that includes anauxiliary load according to another embodiment of the invention.

FIG. 4 is a schematic diagram of a propulsion system according toanother embodiment of the invention.

FIG. 5 is a schematic diagram of a propulsion system according to yetanother embodiment of the invention.

FIG. 6 is a schematic diagram of a propulsion system according to yetanother embodiment of the invention.

FIG. 7 is a schematic diagram of a propulsion system according to yetanother embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of a propulsion system 10 according to anembodiment of the invention. As described in detail below, propulsionsystem 10 may be configured in a pure electric (EV) propulsion systemarrangement that splits power output between two or moreelectromechanical devices or as a hybrid (HEV) propulsion system thatincludes an internal combustion engine in addition to multipleelectromechanical devices. In either an EV or HEV embodiment, theelectromechanical devices are provided on independent channels of amulti-channel DC bus, permitting flexibility in the sizing and operationof the multiple electromechanical devices and increasing the operatingefficiency of the electromechanical devices and overall propulsionsystem.

According to various embodiments, propulsion system 10 is configured tobe incorporated into various types of vehicles including, but notlimited to, automobiles, busses, trucks, tractors, mining equipment,marine craft, and off-road vehicles, including material transportvehicles or personal carrier vehicles, capable of operation both on thesurface and underground such as in mining operations, or other type ofelectrical apparatus such as, for example, a crane, elevator, or lift,as non-limiting examples.

Propulsion system 10 includes an energy storage system 12 and amulti-channel DC bus arrangement having at least two or more independentDC bus channels. In the embodiment illustrated in FIG. 1, themulti-channel DC bus arrangement includes two channels: an A channel 14comprising an A channel DC bus 16 having an A channel positive DC link18 and a B channel 20 comprising a B channel DC bus 22 having a Bchannel positive DC link 24. Energy storage system 12 includes apositive terminal 26 and a negative terminal 28. In one embodiment,energy storage system 12 is a high-voltage or high-power energy storagedevice and may be a battery, a flywheel system, fuel cell, anultracapacitor, or a combination of ultracapacitors, fuel cells, and/orbatteries, as examples. A positive terminal 26 of energy storage system12 is coupled to a first bi-directional DC-DC voltage converter assembly30. In one embodiment, positive terminal 26 of energy storage system 12is also coupled to an optional second bi-directional DC-DC voltageconverter assembly 32 (shown in phantom). As shown, first bi-directionalDC-DC voltage converter 30 is coupled to the B channel positive DC link24 whereas second bi-directional DC-DC voltage converter assembly 32 iscoupled to the A channel positive DC link 18. According to oneembodiment, energy storage system 12 is sized such that secondbi-directional DC-DC voltage converter assembly 32 may be omitted from Achannel 14 resulting in a propulsion system 10 that includes fewer partsand less weight than a system that includes a respective DC-DC voltageconverter on each channel of the multi-channel DC bus assembly.

Both first bi-directional DC-DC voltage converter 30 and secondbi-directional DC-DC voltage converter assembly 32, when used, areconfigured to convert one DC voltage to another DC voltage either bybucking or boosting the DC voltage. Each bi-directional DC-to-DC voltageconverter 30, 32 includes an inductor 34 coupled to a pair of switches36, 38 and coupled to a pair of diodes 40, 42. Each switch 36, 38 iscoupled to a respective diode 40, 42, and each switch/diode pair forms arespective half phase module 44, 46. Switches 36, 38 are shown, forillustrative purposes, as insulated gate bipolar transistors (IGBTs).However, embodiments of the invention are not limited to IGBTs. Anyappropriate electronic switch can be used, such as, for example, metaloxide semiconductor field effect transistors (MOSFETs), silicon carbide(SiC) MOSFETs, Gallium nitride (GaN) devices, bipolar junctiontransistors (BJTs), and metal oxide semiconductor controlled thyristors(MCTs).

The A channel 14 and B channel 20 also include respective DC-to-ACvoltage inverters 48, 50, each of which includes six half phase modules52, 54, 56, 58, 60, and 62 that are paired to form three phases 64, 66,68. Each phase 64, 66, 68 is coupled between a pair of conductors of itsrespective DC bus 22, 16. Specifically, each phase 64, 66, 68 ofDC-to-AC voltage inverter 48 is coupled between A channel positive DClink 18 and an A channel negative DC link 70 of A channel DC bus 16 andeach phase 64, 66, 68 of DC-to-AC voltage inverter 50 is coupled betweenB channel positive DC link 24 and a B channel negative DC link 72 of Bchannel DC bus 22.

An electromechanical device 74 is coupled to DC-to-AC voltage inverter48 on A channel 14. Electromechanical device 74 includes a plurality ofwindings 76, 78, 80 coupled to respective phases 64-68 of DC-to-ACvoltage inverter 48. Propulsion system 10 also includes anelectromechanical device 82 coupled to DC-to-AC voltage inverter 50 onthe B channel 20. As shown, electromechanical device 82 includes aplurality of windings 84, 86, 88 coupled to respective phases 64-68 ofDC-to-AC voltage inverter 50. In one embodiment, electromechanicaldevice 82 is a traction motor and electromechanical device 74 is eitheran alternator or a traction motor. Although the propulsion system 10illustrated in FIG. 1 includes three-phase inverters 48, 50 andthree-phase electromechanical devices 74, 84, it is contemplated thatpropulsion system 10 may utilize any number of phases in alternativeembodiments.

According to one embodiment, electromechanical device 82 and associatedDC-to-AC voltage inverter 50 are sized to provide and accept high powerlevels and operate at higher speeds than electromechanical device 74 andassociated DC-to-AC voltage inverter 48. To minimize system losses,especially during high speed, high power operation, the DC link voltageof B channel 20 may be decoupled from the voltage of energy storagesystem 12 and may be controlled to be at a higher voltage than the DClink voltage of A channel 14. As one non-limiting example,electromechanical device 74 may be designed for a DC link voltage ofapproximately 400 V, with DC-to-AC voltage inverter 48 having switchingdevices rated for approximately 650 V, while electromechanical device 82is designed to operate at a boosted DC link voltage of approximately 630V, with DC-to-AC voltage inverter 50 having switching devices rated for900 V or 1200 V or possibly higher voltage of 1800 V. In addition,because the DC link voltage of the B channel is decoupled from the DClink voltage of the A channel, the operation of DC-to-AC voltageinverter 48 and excitation of electromechanical device 74 may further beoptimized to achieve desired operating characteristics.

Propulsion system 10 also includes a transmission 90 coupled to theoutputs of electromechanical device 74 and electromechanical device 82.Transmission 90 is constructed as a gear assembly, belt assembly, orcombination thereof according to various embodiments. According to oneembodiment, transmission 90 is configured as an electrically variabletransmission (EVT) that couples the outputs of electromechanical devices74, 82 through an arrangement of planetary gears and clutches (notshown). In operation, electromechanical devices 74, 82 may be operatedover a wide range of bi-directional speed, torque, and power commands tominimize power loss and maintain a high degree of overall systemefficiency while operating in either a charge depleting (CD) or chargesustaining (CS) mode of operation.

The output of transmission 90 is coupled to one or more driving wheelsor axles 92 of a vehicle (not shown) through a gear assembly 94, whichmay include a differential. Depending on how the clutches oftransmission 90 are configured, electromechanical device 82 may becoupled to gear assembly 94 through transmission 90 or may be directlycoupled to gear assembly 94 such that the output of electromechanicaldevice 82 bypasses transmission 90.

Propulsion system 10 also includes a controller 96 operably coupled tohalf phase modules 44, 46 of bi-directional DC-DC voltage converters 30,32 by control lines 98. Through appropriate control of switches 36, 38of voltage converters 30, 32, controller 96 is configured to boost avoltage of energy storage system 12 to a higher voltage and to supplythe higher voltage to A channel DC bus 16. Likewise, controller 96 isconfigured to control switching of bi-directional DC-DC voltageconverter 32 to boost the voltage of energy storage system 12 to ahigher voltage and to supply the higher voltage to B channel DC bus 22.In some modes of operation, DC bus 22 may operate at a higher voltagethan DC bus 16. In other modes, DC bus 16 and DC bus 22 may be operatedat the same voltage. Controller 96 is also configured to controlswitches 36, 38 of respective voltage converters 30, 32 to buck avoltage of A channel DC bus 16 and B channel DC bus 22 and to supply thebucked voltages to energy storage system 12.

Controller 96 is also coupled to half phase modules 52-62 of inverters48, 50 by control lines 100. Controller 96, through appropriate controlof half phase modules 52-62 in a motoring mode, is configured to controlinverters 48, 50 to convert the DC voltage or current on respective DCbuses 16, 22 to an AC voltage or current for supply to windings 74-80,84-88 of respective electromechanical devices 74, 82. In a regenerativemode, controller 96 is configured to control half phase modules 52-62 toinvert an AC voltage or current received by respective inverters 48, 50from its corresponding electromechanical device 74, 82 into a DC voltageor current to supply to A channel DC bus 16 or B channel DC bus 22.

In operation, controller 96 receives feedback from a number of sensorsprovided within propulsion system 10 via control lines 102. For example,voltage sensors 104, 106 may be provided on the A channel DC bus 16 andthe B channel DC bus 22, respectively, to allow controller 96 to monitorthe bus voltage of the A and B channels 14, 20. As one skilled in theart will recognize, additional voltage and/or current sensors may beprovided throughout propulsion system 10 to permit controller 96 tomonitor other operating conditions, such as, for example, the chargingvoltage of energy storage system 12. In addition, one skilled in the artwill recognize that controller 96 may receive feedback from and/ortransmit control commands to other components within propulsion system10, such as, for example, internal combustion engine 108.

According to one embodiment propulsion system 10 is configured as a pureelectric vehicle (EV) propulsion system. Alternatively, propulsionsystem 10 is configured in a hybrid electric vehicle (HEV) propulsionsystem and also includes an internal combustion engine 108 (shown inphantom) coupled to transmission 90. According to various embodiments,internal combustion engine 108 may be an internal combustion gasolineengine, an internal combustion diesel engine, an internal combustionengine fueled by natural gas, an external combustion engine, or a gasturbine engine, as examples.

Referring now to FIG. 2, a dual-channel propulsion system 110 isillustrated according to an alternative embodiment that includes avehicle accessory load or auxiliary load 112. Elements and componentscommon to propulsion system 10 and propulsion system 110 are referred toherein with similar part numbering. Similar to propulsion system 10,propulsion system 110 includes an electromechanical device 82 coupled tothe B channel 20 through DC-to-AC voltage inverter 50. Propulsion system110 also includes a bi-directional DC-DC voltage converter 30 configuredto selectively boost a voltage of energy storage system 12 to a busvoltage of B channel DC bus 22 during a motoring mode and buck a voltageB channel DC bus 22 to a voltage of energy storage system 12 during aregenerative or recharging mode.

In addition to B channel 20, the dual-channel DC bus assembly ofpropulsion system 110 includes a second channel or C channel 114 that isconnected to an auxiliary load 112 through a DC-to-AC voltage inverter116, which like DC-to-AC voltage inverter 50, includes six half phasemodules 118, 120, 122, 124, 126, 128 paired to form respective phases130, 132, 134. As illustrated in FIG. 2, C channel 114 includes a Cchannel DC bus 136 having a C channel positive DC link 138 coupled tothe positive terminal 26 of energy storage system 12 through an optionalDC-DC converter 142 (shown in phantom) and a C channel negative DC bus140 coupled to the negative terminal 28 of energy storage system 12.

In one embodiment, energy storage system 12 is sized such that positiveterminal 26 of energy storage system 12 may be directly coupled to Cchannel DC bus 136. Alternatively, an optional bi-directional DC-DCvoltage converter 142 (shown in phantom), similar to bi-directionalDC-DC voltage converter 30 (FIG. 1), is coupled to the C channel DC bus136.

The auxiliary load 112 of propulsion system 110 is coupled to anelectromechanical device 144 having a plurality of windings 146, 148,150, which are coupled to respective phases 130, 132, 134 of DC-to-ACvoltage inverter 116. Auxiliary load may be a pump, heater, cooling fan,electrically powered air conditioning unit a pneumatic or other fluidcompressor unit, as non-limiting examples. In a direct-drive embodiment,electromechanical device 144 is directly coupled to auxiliary load 112.In alternative embodiments including a gear or belt-driven device suchas, for example, a high speed motor running a pump, electromechanicaldevice 144 may be coupled to auxiliary load 112 via an optional gear orbelt assembly component 152 (shown in phantom).

Optionally, propulsion system 110 may include one or more electricalauxiliary loads 154 (shown in phantom) coupled to the C channel positiveand negative DC links 138, 140. According to an exemplary embodiment,electrical auxiliary load(s) 154 may include high-power electrical loadssuch as, for example, resistive heating elements.

Propulsion system 10 (FIG. 1) and propulsion system 110 (FIG. 2) aredescribed above as including two channels, an A channel 14 and B channel20 both coupled to a transmission 90 in the case of propulsion system 10and an A channel 14 coupled to a transmission 90 and a C channel 114coupled to an auxiliary load 112 in the case of propulsion system 110.Alternative embodiments may include more than two DC bus channels, withtwo or more channels coupled to the vehicle transmission and one or moreDC bus channels coupled to auxiliary loads. As one example, thepropulsion system 156 of FIG. 3 includes A channel 14 and B channel 20coupled to transmission 90 via respective electromechanical devices 74,82 and C channel 114 coupled to auxiliary load 112 via electromechanicaldevice 144. In the embodiment shown, the positive DC links 18, 24, 138of each channel are coupled to the positive terminal 26 of energystorage system 12. As described with respect to FIGS. 1 and 2,bi-directional DC-DC voltage converter assembly 32 of A channel 14and/or bi-directional DC-DC voltage converter 142 of C channel DC bus136 are optional components and may be omitted based on the sizing ofenergy storage system 12.

FIG. 4 is a schematic diagram of a propulsion system 158 according toanother embodiment of the invention. Elements and components common topropulsion systems 10 and 158 will be discussed relative to the samereference numbers as appropriate. In addition to the components commonwith propulsion system 10, propulsion system 158 differs from propulsionsystem 10 in that it includes a first energy storage system 160 and asecond energy storage system 162 instead of the single energy storagesystem 12 of FIG. 1.

As shown in FIG. 4, first and second energy storage systems 160, 162 areconnected by a common negative DC link 164 coupled to the respectivenegative terminals 166, 168 of the energy storage systems 160, 162. Thepositive terminal 170 of first energy storage system 160 is coupled tothe A channel positive DC link 18 through DC-DC converter 32 and thepositive terminal 172 of second energy storage system 162 is coupled tothe B channel positive DC link 24 through DC-DC converter 30.

According to one embodiment, first energy storage system 160 is a highspecific-power energy storage device and second energy storage system162 is a high specific-energy storage device. First energy storagesystem 160 may be, for example, an ultracapacitor having multiplecapacitor cells coupled to one another, where the capacitor cells mayeach have a capacitance that is greater than approximately 500 Farads.Alternatively, first energy storage system 160 may be a high powerbattery having a specific-power of approximately 350 W/kg or greater ora combination of one or more ultracapacitors and batteries. Inembodiments where first energy storage system 160 is an ultracapacitor,first bi-directional DC-DC voltage converter 32 is included on the Achannel 14. Alternatively, where first energy storage system 160 is abattery, optional first bi-directional DC-DC voltage converter 32 (shownin phantom) may optionally be omitted based on the sizing of firstenergy storage system 160.

Second energy storage system 162 has a relatively low specific power ascompared with first energy storage system 160. As used herein, lowspecific power describes an energy storage device demonstrated toachieve a specific power on the order of approximately 200 W/kg orlower. According to various embodiments, second energy storage system162 may be, for example, a high specific energy battery or high energydensity battery. The term energy battery used herein describes a highspecific energy battery or high energy density battery demonstrated toachieve an energy density on the order of 100 W-hr/kg or greater (e.g.,a Li-ion, sodium-metal halide, sodium nickel chloride, sodium-sulfur,Li-Air, or zinc-air battery).

In one embodiment, second energy storage system 162 has a relativelyhigh resistivity and impedance as compared with first energy storagesystem 160. In another embodiment, the relatively low specific power ofsecond energy storage system 162 may be due to an imbalance of theindividual battery cells comprising the energy storage system. In oneembodiment, second energy storage system 162 is a low-cost lithium ionbattery. Alternatively, second energy storage system 162 may be a sodiummetal halide battery, a sodium sulfur battery, a nickel metal halidebattery, a Zinc-air battery, a lead acid battery, and the like.

In embodiments where first energy storage system 160 is configured as apower battery, propulsion system 158 may be incorporated into a transitbus, as an example. In yet another embodiment, first and second energystorage systems 160, 162 are both configured as high specific-energystorage devices.

By configuring propulsion system 158 with a separate energy storagesystem for each channel of the multi-channel DC bus assembly (e.g.,first energy storage system 160 for the A channel 14 and second energystorage system 162 for the B channel 20), the energy storage systems160, 162 may be individually sized for its respective channel tominimize the size, weight, and cost of the overall propulsion system 158and account for the differing roles that the electromechanical devices74, 82 may play in the propulsion system 158. For example,electromechanical device 74 may be operated to provide high power duringacceleration periods whereas electromechanical device 82 may be operatedto provide a longer-lasting power to the vehicle to increase a travelingdistance thereof. Accordingly, because the peak power of the A channel14 may be two or more times greater than the peak power of B channel 20,the first and second energy storage systems 160, 162 may be individuallysized accordingly.

Control over the DC link voltage for each respective channel is providedby the respective bi-directional DC-DC voltage converter 30, 32. Inaddition, propulsion system 158 may be controlled to further optimizeefficiency and cost while meeting peak power demands during operationand managing usable energy from first and second energy storage systems160, 162 for various vehicle drive cycles. The inclusion of separatefirst and second energy storage systems 160, 162 may also lead toadditional savings in the power electronics and passive componentsincluded the first and/or second bi-directional DC-DC voltage converters30, 32 as well as the power electronic modules and passive components inone or both of the energy storage systems 160, 162. In addition, theinclusion of a separate energy storage system for each channel mayresult in improved operating efficiency by permitting the one or more ofthe energy storage systems 160, 162 to be operated at a lower voltagethan a system that includes a single energy storage system sized to meetthe peak power demands of multiple channels, such as energy storagesystem 12 of FIG. 1 that is sized to meet the peak power demand of boththe A and B channels 14, 20.

Optionally, propulsion system 158 includes a switching element orcoupling device 174 (shown in phantom) positioned between the positiveDC links 18, 24 of A channel 14 and B channel 20. According to variousembodiments, coupling device 174 may be constructed as anelectromechanical switching device, a solid-state type switching device,a diode, a combination of a resistor and a contactor or solid-stateswitch, as non-limiting examples. Controller 96 is connected to couplingdevice 174 via control lines 176 and operates coupling device 174 in anopen state or a closed state so that the A and B channels 14, 20 may beoperated at different DC link voltages during certain modes of operationand selectively coupled together to operate at the same DC link voltageduring other operating modes. For example, during a motoring mode ofoperation, coupling device 174 may be configured in an open state topermit the A channel 14 to operate at a lower DC link voltage than the Bchannel 20 so that electromechanical device 82 can run efficiently at ahigher operating voltage than electromechanical device 74. In addition,during a regenerative braking event or during an engine recharging eventof one of the energy storage systems 162, 160, controller 96 may beconfigured to close coupling device 174 such that AC voltage or currentgenerated by electromechanical device 74, inverted into a DC voltage orcurrent by DC-to-AC voltage inverter 48 and supplied to secondbi-directional DC-DC voltage converter assembly 30 to recharge secondenergy storage system 162.

Referring now to FIG. 5, a propulsion system 178 is illustratedaccording to another embodiment of the invention. Elements andcomponents of propulsion system 178 common to propulsion systems 10, 158will be discussed relative to the same reference numbers as appropriate.

Similar to propulsion system 158 of FIG. 4, propulsion system 178includes a first energy storage system 180 and a second energy storagesystem 182, which may be individually sized based on the peak powerdemand of its respective channel 14, 20 thereby permitting a reductionin the size, overall weight, and cost of the propulsion system. Inaddition, first energy storage system 180 may be selected to have ahigher specific power (and lower internal impedance) than second energystorage system 182, which may be sized for a higher nominal voltage thanfirst energy storage system 180. Accordingly, in one embodiment, firstenergy storage system 180 is configured as a high specific-power energystorage device similar to first energy storage system 160 (FIG. 4) andsecond energy storage system 182 is configured as a high specific-energystorage device similar to second energy storage system 162 (FIG. 4).

Unlike the configuration of propulsion system 158 of FIG. 4, however,first energy storage system 180 and second energy storage system 182 ofpropulsion system 178 are arranged in a series configuration, as shownin FIG. 5, with the positive terminal 184 of first energy storage system180 coupled to the negative terminal 186 of second energy storage system182. As shown, the positive terminal 188 of second energy storage system182 is connected to first bi-directional DC-DC voltage converter 30 andthe negative terminal 190 of first energy storage system 180 isconnected to A channel negative DC link 70 and also connected to Bchannel negative DC link 72. By connecting first and second energystorage systems 180, 182 in series, the power sharing between energystorage systems 180, 182 is a function of the relative voltages of thetwo energy storage systems. In other words, the power out of each energystorage system 180, 182 is a function of the voltage of the respectiveenergy storage system 180, 182 as a result of the series configuration.Because the series connection of energy storage systems 180, 182 allowsthe relative voltages of the two systems to be summed, energy storagesystems 180, 182 may be sized to have lower voltages than a propulsionsystem with a single energy storage system with a comparable overallvoltage output, such as that shown in FIG. 1.

In operation, the state of charge (SOC) of both first energy storagesystem 180 and second energy storage system 182 is maintained withinprescribed thresholds and within a prescribed operating range via theswitching commands that the controller 96 transmits to the first andsecond bi-directional DC-DC voltage converter 30, 32. Independentcontrol over the DC link voltages of the A channel 14 and B channel 20are provided by bi-directional DC-DC voltage converter 30, 32 andassociated loads from DC-to-AC voltage inverters 48, 50 andelectromechanical devices 74, 82.

Optionally, propulsion system 178 may also include optional couplingdevice coupling device 174 (shown in phantom) positioned between thepositive DC links 18, 24 of A channel 14 and B channel 20. Couplingdevice 174 may be operated by controller 96 in a similar manner inpropulsion system 178 as described above with respect to propulsionsystem 158, so that the DC link voltages of the A and B channels 14, 20are the same during certain modes of operation and differ during otheroperating modes.

FIG. 6 illustrates a propulsion system 192 according to yet anotherembodiment of the invention. As propulsion system 192 includes many ofthe same components and elements of propulsion system 178 of FIG. 5,elements and components common to systems 178 and 192 will be discussedrelative to the same reference numbers as appropriate.

In addition to the components common with propulsion system 178,propulsion system 192 includes a coupling device 194 that replaces thebi-directional DC-DC voltage converter 32 of propulsion system 178 onthe A channel 14 and that is controlled by controller 96 via controllines 196. Similar to coupling device 174, coupling device 194 may beconstructed as an electromechanical switching device, a solid-state typeswitching device, a diode, a combination of a resistor and a contactoror solid-state switch, as non-limiting examples. Because coupling device194 is a lower cost component than a bi-directional DC-DC voltageconverter, propulsion system 192 may be manufactured at a lower costthan propulsion system 178 of FIG. 5. Further, because coupling device194 operates at a higher efficiency than a bi-directional DC-DC voltageconverter, replacing the A channel bi-directional DC-DC voltageconverter 32 of FIG. 1 with coupling device 194 increases the overallefficiency of propulsion system 192.

Similar to propulsion system 178 of FIG. 5, propulsion system 192includes first and second energy storage systems 180, 182 arranged inseries such that positive terminal 184 of first energy storage system180 is coupled to negative terminal 186 of second energy storage system182. First energy storage system 180 is sized based on the desiredacceleration of the propulsion system 192 whereas second energy storagesystem 182 is sized based on the desired distance of travel whileoperating using the electric drive. In the embodiment illustrated inFIG. 6, first energy storage system 180 is configured as a powerbattery, rather than an ultracapacitor, in order to maintain a DC linkvoltage for DC-to-AC voltage inverter 48 within a threshold value of apredetermined nominal voltage when coupling device 194 is closed. Inaddition, the nominal voltage of first energy storage system 180 isselected such that the normal operating voltage range of first energystorage system 180 is within a threshold value of the optimized DC linkvoltage for the A channel 14 that is determined based on the designparameters of electromechanical device 74.

Referring to FIG. 7, a dual-channel propulsion system 198 is shownaccording to yet another embodiment. Elements common to propulsionsystems 10, 198 are referred to with similar numbering. Similar topropulsion system 10 of FIG. 1, propulsion system 198 includes a singleenergy storage system 200 coupled to both the A channel 14 and the Bchannel 20. Similar to energy storage system 12 (FIG. 1), energy storagesystem 200 may be a high-voltage or high-power energy storage device andmay be a battery, a flywheel system, fuel cell, an ultracapacitor, or acombination of ultracapacitors, fuel cells, and/or batteries, accordingto various embodiments. As shown, the positive terminal 202 of energystorage system 200 is coupled to B channel positive DC link 24 throughfirst bi-directional DC-DC voltage converter 30. The A channel positiveDC link 18 is coupled to the positive terminal 202 of through secondbi-directional DC-DC voltage converter assembly 32.

An optional bypass contactor 204 (shown in phantom) is included on the Achannel 14 between energy storage system 200 and DC-to-AC voltageinverter 48. According to various embodiments, bypass contactor 204 maybe constructed as an electromechanical switching device, a solid-statetype switching device, a diode, a combination of a resistor and acontactor or solid-state switch, as non-limiting examples.

In one embodiment, energy storage system 200 is sized such that positiveterminal 202 of energy storage system 200 may be directly coupled to Achannel positive DC link 18 through an optional bypass contactor.Alternatively, an optional bi-directional DC-DC voltage converterassembly 32 (shown in phantom) is coupled to the A channel DC bus 16. Inthis embodiment, at least one of optional DC-DC converter 32 or optionalbypass contactor 204 is utilized.

Controller 96 is connected to bypass contactor 204 via control lines206. In certain operating modes controller 96 operates bypass contactor204 in an open state, thereby creating a power flow path between energystorage system 200 and DC-to-AC voltage inverter 48 that travels throughoptional bi-directional DC-DC voltage converter assembly 32, which iscontrolled by controller 96 to boost the voltage of energy storagesystem 200 to the voltage of the A channel DC bus 16. In other operatingmodes, controller 96 operates the optional bypass contactor 204 in aclosed state to create a power flow path between energy storage system200 and DC-to-AC voltage inverter 48 that bypasses optionalbi-directional DC-DC voltage converter assembly 32. When bypass couplingdevice 204 is operated in a closed state to bypass bi-directional DC-DCvoltage converter assembly 32, propulsion system 198 experiences animprovement in efficiency over simply turning off chopping because thelosses in bi-directional DC-DC voltage converter assembly 32 areeliminated.

As described above, embodiments of the invention utilize a multi-channelDC bus assembly configured to permit the individual DC bus channels tooperate either at a common voltage or at different voltages while beingconnected to one or multiple common energy storage systems. In someembodiments, the outputs of electromechanical devices coupled torespective channels of the DC bus assembly are coupled together througha common transmission assembly, such as an electrically variabletransmission. Together, the electromechanical devices, DC-AC inverters,and bi-directional DC-DC converters associated with each channel of themulti-channel DC bus assembly along with the energy storage system(s)operate over a wide range of bi-directional speed, torque, and powerthat may be controlled to minimize power loss and maintain a high degreeof overall system efficiency when the energy storage system(s) areoperating in charge depleting and charge sustaining modes. Optionally, aheat engine may be coupled to the transmission assembly to maintaincharge on the energy storage system(s) and an output of one or more ofthe DC bus channels may be coupled to an auxiliary load. Beneficially,the propulsion systems disclosed herein enable the energy storagesystem(s) to be designed to minimize size, weight, and cost whileproviding improved efficiency through independent control of the DC linkvoltage on each of the DC bus channels of the multi-channel DC busassembly.

A technical contribution for the disclosed apparatus is that it providesfor a controller implemented technique for boosting a voltage of anenergy storage system to a boosted voltage and supplying the boostedvoltage to a voltage bus.

According to one embodiment of the invention, an apparatus includes amulti-channel DC bus assembly comprising a first channel and a secondchannel, a first electromechanical device coupled to a positive DC linkof the first channel, and a second electromechanical device coupled to apositive DC link of the second channel. A first DC-to-AC voltageinverter is coupled to the positive DC link of the first channel and asecond DC-to-AC voltage inverter is coupled to the positive DC link ofthe second channel. The apparatus further includes a bi-directionalvoltage modification assembly coupled to the positive DC link of thesecond channel and a first energy storage system electrically coupled tothe first electromechanical device.

In accordance with another embodiment of the invention, a method offabricating a propulsion system includes coupling a first DC-to-ACvoltage inverter to a first voltage bus, coupling a firstelectromechanical device to the first DC-to-AC voltage inverter, andcoupling a second DC-to-AC voltage inverter to a second voltage bus. Themethod also includes coupling a second electromechanical device to thesecond DC-to-AC voltage inverter, coupling a bi-directional DC-DCvoltage converter to the second voltage bus, coupling a first energystorage system to the bi-directional DC-DC voltage converter, andprogramming a controller to control switching of the bi-directionalDC-DC voltage converter to boost a voltage of the first energy storagesystem to a boosted voltage different from a voltage of the firstvoltage bus.

In accordance with yet another embodiment of the invention, a vehiclepropulsion system includes a DC bus assembly having a first DC bus and asecond DC bus. The vehicle propulsion system also includes a firstbi-directional DC-to-DC voltage converter coupled to the first DC bus, ahigh specific-power energy storage device coupled to a low voltage sideof the first bi-directional DC-to-DC voltage converter, a firstelectromechanical device coupled to the first DC bus through a firstDC-to-AC voltage converter, and a second electromechanical devicecoupled to the second DC bus through a second DC-to-AC voltageconverter. A controller is programmed to control the firstbi-directional DC-to-DC voltage converter to boost a voltage of thefirst electromechanical device to a boosted voltage and supply theboosted voltage to the first DC bus, the boosted voltage different thana voltage of the second DC bus.

In accordance with yet another embodiment of the invention, a vehiclepropulsion system includes a first electromechanical device coupled to apositive DC link of a first DC bus, an auxiliary load coupled to anoutput of the first electromechanical device and a first DC-to-ACvoltage inverter coupled to the positive DC link of the first DC bus. Asecond electromechanical device is coupled to a positive DC link of asecond DC bus and a transmission is coupled to an output of the secondelectromechanical device. The vehicle propulsion system also includes asecond DC-to-AC voltage inverter coupled to the positive DC link of thesecond DC bus, an energy storage system electrically coupled to thepositive DC link of the second DC bus, and a bi-directional voltagemodification assembly coupled to the positive DC link of the second DCbus and configured to boost a voltage of the first energy storage systemto a voltage different from a voltage of the first DC bus.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

What is claimed is:
 1. An apparatus comprising: a multi-channel DC busassembly comprising a first channel and a second channel; a firstelectromechanical device coupled to a positive DC link of the firstchannel; a second electromechanical device coupled to a positive DC linkof the second channel; a first DC-to-AC voltage inverter coupled to thepositive DC link of the first channel; a second DC-to-AC voltageinverter coupled to the positive DC link of the second channel; abi-directional voltage modification assembly coupled to the positive DClink of the second channel; and a first energy storage systemelectrically coupled to the first electromechanical device.
 2. Theapparatus of claim 1 wherein the first energy storage system is furthercoupled to the second electromechanical device.
 3. The apparatus ofclaim 1 further comprising a second energy storage system electricallycoupled to the second electromechanical device.
 4. The apparatus ofclaim 3 wherein the first energy storage system comprises at least oneof an ultracapacitor and a power battery; and wherein the second energystorage system comprises an energy battery.
 5. The apparatus of claim 3wherein a negative terminal of the first energy storage system iscoupled to a negative terminal of the second energy storage system. 6.The apparatus of claim 3 wherein the first energy storage system and thesecond energy storage system are coupled together in a seriesarrangement.
 7. The apparatus of claim 1 further comprising abi-directional voltage modification assembly coupled to the positive DClink of the first channel.
 8. The apparatus of claim 1 furthercomprising a coupling device configured to selectively couple thepositive DC link of the first channel to the positive DC link of thesecond channel.
 9. The apparatus of claim 8 further comprising acontroller programmed to: close the coupling device during at least oneof a regenerative braking mode of operation and a charge sustaining modeof operation; and open the coupling device during a motoring mode ofoperation.
 10. The apparatus of claim 1 further comprising a couplingdevice configured to selectively couple the positive DC link of thefirst channel to the energy storage device.
 11. The apparatus of claim 1further comprising at least one of a gear assembly and a belt assemblyconfigured to mechanical couple the first electromechanical device tothe second electromechanical device.
 12. The apparatus of claim 1wherein the multi-channel DC bus assembly further comprises a thirdchannel having an auxiliary load coupled thereto.
 13. A method offabricating a propulsion system comprising: coupling a first DC-to-ACvoltage inverter to a first voltage bus; coupling a firstelectromechanical device to the first DC-to-AC voltage inverter;coupling a second DC-to-AC voltage inverter to a second voltage bus;coupling a second electromechanical device to the second DC-to-ACvoltage inverter; coupling a bi-directional DC-DC voltage converter tothe second voltage bus; coupling a first energy storage system to thebi-directional DC-DC voltage converter; and programming a controller tocontrol switching of the bi-directional DC-DC voltage converter to boosta voltage of the first energy storage system to a boosted voltagedifferent from a voltage of the first voltage bus.
 14. The method ofclaim 13 further comprising coupling a second energy storage system inseries with the first energy storage system.
 15. The method of claim 13further comprising coupling a negative terminal of a second energystorage system to a negative terminal of the first energy storagesystem.
 16. The method of claim 13 further comprising: coupling a secondbi-directional voltage converter to the first voltage bus; and couplinga second energy storage system to the second bi-directional voltageconverter.
 17. The method of claim 13 further comprising: coupling aswitching device between the first voltage bus and the second voltagebus; and programming the controller to open the switching device in afirst operating mode and close the switching device in a secondoperating mode.
 18. A vehicle propulsion system comprising: a DC busassembly comprising: a first DC bus; and a second DC bus; a firstbi-directional DC-to-DC voltage converter coupled to the first DC bus; ahigh specific-power energy storage device coupled to a low voltage sideof the first bi-directional DC-to-DC voltage converter; a firstelectromechanical device coupled to the first DC bus through a firstDC-to-AC voltage converter; a second electromechanical device coupled tothe second DC bus through a second DC-to-AC voltage converter; and acontroller programmed to control the first bi-directional DC-to-DCvoltage converter to boost a voltage of the first electromechanicaldevice to a boosted voltage and supply the boosted voltage to the firstDC bus, the boosted voltage different than a voltage of the second DCbus.
 19. The vehicle propulsion system of claim 18 further comprising asecond bi-directional DC-to-DC converter coupled to the second channel,the second bi-directional DC-to-DC converter having a voltage ratinglower than a voltage rating of the first bi-directional DC-to-DCconverter.
 20. The vehicle propulsion system of claim 19 a highspecific-energy energy storage device coupled to a low voltage side ofthe second bi-directional DC-to-DC converter.
 21. The vehicle propulsionsystem of claim 18 further comprising a high specific-energy energystorage device coupled in series with the high specific-power energystorage device.
 22. The vehicle propulsion system of claim 18 furthercomprising a high specific-energy energy storage device having anegative terminal coupled to a negative terminal of the highspecific-power energy storage device.
 23. The vehicle propulsion systemof claim 18 further comprising a transmission coupled to an output ofthe first electromechanical device and an output of the secondelectromechanical device.
 24. A vehicle propulsion system comprising: afirst electromechanical device coupled to a positive DC link of a firstDC bus; an auxiliary load coupled to an output of the firstelectromechanical device; a first DC-to-AC voltage inverter coupled tothe positive DC link of the first DC bus; a second electromechanicaldevice coupled to a positive DC link of a second DC bus; a transmissioncoupled to an output of the second electromechanical device; a secondDC-to-AC voltage inverter coupled to the positive DC link of the secondDC bus; an energy storage system electrically coupled to the positive DClink of the second DC bus; and a bi-directional voltage modificationassembly coupled to the positive DC link of the second DC bus andconfigured to boost a voltage of the first energy storage system to avoltage different from a voltage of the first DC bus.
 25. The vehiclepropulsion system of claim 24 wherein the energy storage system iselectrically coupled to the positive DC link of the first DC bus. 26.The vehicle propulsion system of claim 24 further comprising a secondbi-directional voltage modification assembly coupled to the positive DClink of the first DC bus.
 27. The vehicle propulsion system of claim 24further comprising an electrical auxiliary load directly coupled to thepositive DC link of the first DC bus.
 28. The vehicle propulsion systemof claim 24 further comprising: a third electromechanical device coupledto a positive DC link of a third DC bus; and wherein an output of thethird electromechanical device is coupled to the transmission.